This invention relates to methods and systems for the fabrication of optical elements of precise length and more particularly to fabrication of elements having optical path lengths exact enough to be used in applications for optical communications which use optical interferometry.
There have long been needs for exactness in mechanical and optical devices and systems, and these needs have heretofore been met by a variety of techniques, from mechanical to optical. Examples of the latter are found in a 1922 publication of the Department of Commerce, entitled xe2x80x9cInterference Methods for Standardizing and Testing Precision Gage Blocksxe2x80x9d by C. G. Peters and H. S. Boyd, which describes light wave interference methods which are not subject to the xe2x80x9cappreciable errorsxe2x80x9d found with xe2x80x9cmicrometer microscopesxe2x80x9d and xe2x80x9ccontact instrumentsxe2x80x9d. The authors describe optical interference approaches imparting about an order of magnitude improvement, e.g. from 0.25 to 0.025 microns. While directed to the calibration of gage blocks, this article nonetheless evidences what even today must be acknowledged as an ingenious optical approach to ascertaining the dimensions, flatness and parallelism of gage surfaces.
In modern telecommunication systems, however, dimensional measurements are embedded in a number of other factors which arise from the way in which optical elements are used. In telecommunications systems using device wavelength division multiplexing (DWDM), for example, polarization interferometry that requires precise differential delays between different beams is used in generating a required filter function. Interleavers employing these relationships are described in U.S. application Ser. No. 09/898,469 referenced above to provide athermal operation within individual stages of multi-stage multiplexers and demultiplexers.
The optical path length of an optical component through which light traverses is dependent not only on distance but also on intrinsic properties, such as the index of refraction, of the components. Modern optical systems must meet such demanding specifications that optical path length has become an important consideration. The fabrication of interleaving optical filters for DWDM using athermal delay line interferometers requires strict control of the optical path lengths of the glass elements. This control is necessary to meet the tight tolerances on absolute channel frequencies for DWDM applications. Traditional physical path length measurements such as mechanical or non-contact thickness probes with sub-micron accuracies are thus not adequate because the optical path length depends on both physical thickness and the absolute index of refraction of the medium. Also, for individual glass melts the index of refraction of typical optical glasses varies by 10xe2x88x925 to 10xe2x88x924 from a nominal or target value despite best production methods. This variation introduces substantial uncertainty in optical path length even if the physical thickness of the glass is known exactly. A particular additional requirement is that the optical path length of any xe2x80x9cglass windowxe2x80x9d must be accurately measured at a chosen wavelength of operation (e.g., 1550 nm) to account for material dispersion.
Methods and systems for fabricating optical elements such as microoptic elements used in introducing differential delays in DWDM interleavers, use a number of different measurements of frequency periodicity at successive evolutionary processing steps leading to final sizing. The method and system provide precise frequency periodicities, with optical elements being so interrelated as to be acceptably athermal at a chosen frequency. The optical elements thus form the basis for a desired transmission spectrum for a DWDM interleaver. Frequency periodicity is synthesized by measuring output amplitudes derived from differential delays of a test beam at a plurality of incrementally varying wavelengths in the wavelength range of interest, using polarization interferometers to introduce a filter function.
To fabricate to interleaver precision, the optical frequency response of the interleaver must be characterized to sub-GHz in terms of accuracy. The first step in the characterization process is to measure the temperature dependence of the glass, and then calculate to high accuracy the physical lengths needed for both athermal operation and desired frequency periodicity. A first optical element or window is then ground and polished to the desired frequency periodicity given by a first calculation. For example, consider the case of a 50 GHz interleaver with a 100 GHz free spectral range (FSR). The first window is fabricated from the higher index glass of a pair of glasses to be used and is polished to within about 0.04 GHz of the target value to ensure good temperature insensitivity of the final interleaver. This glass element is left in the optical frequency measurement system, and the second window is then ground, polished and repeatedly measured by placing into the second arm of the optical path length measurement system until the frequency periodicity of the combined two glass interleaver is 100.00+xe2x88x920.03 GHz. A like process is utilized to fabricate time delay elements for other FSR""s, such as 25 GHz to 200 GHz.
In accordance with the invention, the frequency periodicity is ascertained during different steps using the differential delays introduced between different optical elements or one optical element and air in the delay paths of a polarization interferometer. An input test beam is propagated through both delay paths, but varied incrementally in wavelength through a selected range of wavelengths. This results in derivation of a sinusoidal variation from which peak to peak spacings determinative of frequency period can be calculated so that optical path length correction can be computed to a degree of accuracy dependent on the state of dimensional refinement of the element. By starting with precursor elements large enough in transverse area for multiple microoptic elements, and using the given oversize in thickness in the precursors, removal of thickness to final dimension can suffice for all microoptic elements at the same time.
To achieve these tolerances, the measurement system described herein meets the extremely high accuracy standards implicitly required for the measurement of optical path length. During the final polishing process of the glass window blanks, the optical path length is periodically measured until the target value is achieved. This measurement technique enables conventional polishing techniques to achieve the desired thicknesses.
Precise optical path length measurements have been achieved, accurate to better than 10 ppm. That is, a glass element with a free spectral range of 100.000 GHz can be fabricated to have a period accurate to better than 1 MHz. The parameter that is directly measured is the optical frequency response of the interleaver in about the 1500 to 1600 nm wavelength range but the determinative result is the establishment of optical path length.
A measurement system in accordance with the invention employs a tunable laser, controlled by a data processor to scan a selected wavelength range in equal, small increments, to generate wavelength varying test beams. The beams are directed through a differential delay system using polarization interferometry to generate a wavelength dependent output. This is received at a spectrum analyzer which stores the sinusoidally varying amplitude readings from the different wavelengths for analysis.
The data processor receives the data and employs a least squares fit program to analyze the sinusoidal variations and ultimately derives the length correction needed for an optical element. The optical measurement apparatus for introducing differential delays in microoptic elements, which area used in testing temperature dependence is in the form of a single stage interleaver. Measurement apparatus for large precursor elements incorporates stages which can be adjusted in two dimensional to position the optical element. Additionally, using illumination directed from a broadband light source through the optical element onto the spectrum analyzer, the tilt and tip orientation of the optical element can be optimized before differential delay readings are made. The laser beam power is advantageously monitored by a power meter coupled to provide measurement signals to the data processor for use in equalizing readings derived during scanning. Also a polarization scrambler is preferably employed in the beam path where polarization dependence in the interferometer may affect readings, by assuring that there is no dominant polarization.